This is probably the simplest idea to generate flashing light from an LED using AC. The circuit is relatively the simple way of flashing one or more LEDs from a high voltage DC obtained from Mains. This can be used as a Mains indicator or Mock flasher.
The circuit uses a diac for the alternate switching of LED. The diac is usually used in pulse generator circuits to trigger SCR and Triac. If a low voltage passes through a diac, it simply behaves like an open circuit and only very low current passes through it. But if the voltage increases to the breakdown threshold of the diac, it will pass heavy current. Usually 35 volt DC is required to attain the threshold level of diac. Unlike SCR, diac conduct in both the directions. In the circuit, a commonly available DB3 diac is used. Diode D1 rectifies AC and generates a high volt DC. Resistor R1 safely controls the DC to operate diac and LED.
Normally LED will be OFF. When the capacitor charges fully, diac gets the threshold voltage and fires. This provides current to LED and it lights. Resistor R2 makes the LED current to a safer value of 30 mA. When the diac conducts, C1 discharges and again the breakdown voltage of diac decreases and LED turns off. Thus the charging/discharging cycles of C1 makes the LED flashing. The value of C1 determines the flash rate. Higher values give slow flash rate and vice versa. If the threshold level of diac is not obtained using the given value of R1, reduce it to 10K, but its wattage should be increased to 5 watts.
Caution: The circuit is directly connected to high volt AC and there is no galvanic isolation. Take utmost care while handling the circuit. Enclose it in a shock proof case. Do not touch any points when it is connected to Mains.